Millimeter wave automotive radar of various types have been developed and utilized in different forms in the prior art. Recently, there has been considerable development in the application of millimeter wave radar to the automotive application known as Adaptive Cruise Control (ACC). ACC uses forward looking sensors mounted on an automobile to collect information about objects in the roadway ahead of the automobile, and transmit the information to the driver. Examples of information collected are data on velocity, direction and distance of objects within the detection range of the sensors.
It is common in prior art applications of millimeter wave radar to ACC, to employ a beam or a plurality of beams to scan an azimuthal field of view ahead of the vehicle on which the radar is mounted.
Scanning the azimuthal field of view using millimeter wave radar has been performed by both mechanical and electrical means in the prior art. Mechanical scanning radar however necessitates unduly large and precise structures to accommodate the mechanical means to accomplish a scan of an azimuthal field of view.
Electronic beam switching radar performs a scan of an azimuthal field of view by electronically varying the direction of radiated beams from an antenna array. By varying the directionality of the radiated beam by electronic means, electronic beam switching radar eliminates the mechanical elements needed to perform the task of scanning an azimuthal field of view and thereby reduces the size, complexity and cost of the structure needed to house the ACC radar.
The reduction in size of an ACC radar such as that provided in the electronic beam switching context is desirable for several reasons. Space is at a premium in the body structures of automobiles and the smaller a device is, the easier it is to place unobtrusively within, or on the structure of the vehicle. Moreover, a reduction in size and complexity of a device often renders the device less costly to produce in large numbers.
Among various examples of electronic beam switching radar in the prior art, a self phased (or self steered) antenna array has been used to change the direction of beams radiated from the array. The change in direction of radiated beams is accomplished by distinct phase differences between adjacent antenna elements. The radiating elements of the antenna can be used as a phased array antenna by simply setting a phase difference between antenna elements.
Varying the directivity in this manner however, can prove to be problematic depending on, for example, the size and distance (from the antenna) of objects to be detected. Moreover, unwanted radiation from the feeders to the antenna array, which are typically formed on the same planar surface, can lead to the deterioration of the directivities of the steered beams and worsened sidelobe levels